1. Field of the Invention
The present invention relates generally to the field of bookbinding and in particular to apparatus for preparing a stack of sheets to be bound for binding.
2. Description of Related Art
Bookbinding apparatus have been developed which permits stacks of sheets to be bound using thermally activated adhesive binder strips. Such binder strips are typically applied using relatively low cost desktop binding machines such as disclosed in U.S. Pat. No. 5,052,873, the contents of which are also incorporated herewith by reference. Referring to the drawings, FIG. 1 shows a binder strip 20 disposed adjacent the insertion point 30A of a conventional binding machine 30. A user first inserts a stack of sheets 32 to be bound in an upper opening of the machine. Controls 30B are then activated to commence the binding process. The binding machine operates to sense the thickness of the stack 32 and indicates on a machine display 30C the width of binder strip 20 to be used. Typically, three widths can be used, including wide, medium and narrow. The binder strip includes a flexible substrate 20A having a length that corresponds to the length of the edge of the stack 32 to be bound and a width somewhat greater than the thickness of the stack. A layer of heat-activated adhesive is formed on one side of the substrate, including a low viscosity, low tack central adhesive band 20C and a pair of high viscosity, high tack outer adhesive bands 20B.
Once the user has selected the binder strip of appropriate width, the user manually inserts the strip 20 into the strip loading port 30A of the machine. The end of the strip, which is positioned with the adhesive side up, is sensed by the machine and is drawing into the machine using an internal strip handling mechanism. The machine then operates to apply the strip to the edge of the stack to be bound. The strip is essentially folded around the edge of the stack, with heat and pressure being applied so as to activate the adhesives. Once the adhesives have cooled to some extent, the bound book is removed from the binding machine so that additional books can be bound. FIG. 2 depicts a partial end view of the bound stack 32. As can be seen, the substrate 20A is folded around the bound edge of the stack. The high tack, high viscosity outer adhesive bands 20B function to secure the strip to the front and back sheets of the stack. These sheets function as the front and rear covers and can be made of heavy paper or the like. The central, low viscosity adhesive 20C functions to secure the individual sheets of the stack by flowing up slightly between the sheets during the binding process.
Although the above-described binding technique provides a reliable bind in most applications, problems arise when the sheets of the stack have special coatings. Such coatings are applied to the sheets for various purposes to enhance the characteristics of the sheet, such as improving the ability of the sheet to receive special printing inks. In any event, such coatings very frequently prevent the central adhesive 20C from adhering adequately to the individual sheets of the stack. This results in an unsatisfactory bind where sheets frequently separate from the stack. Various approaches have been used to address this problem. One approach is to use different types of adhesive for the central adhesive 20C. Another approach is to texturize the stack of sheets prior to binding so that the adhesive is more likely to accept the central adhesive. By way of example, in U.S. Pat. No. 5,961,268 entitled “Method and Device for Adhesive Binding of Printed Products”, a rotating wire brush is applied to the edge of a stack of sheets prior to binding. This approach has not been found satisfactory in addressing the problems relating to coated papers. As a further example, prior art binding systems commonly referred to as perfect binding incorporate milling apparatus that grinds or mills the edge of a stack to be bound. However, stacks of coated sheets processed in this manner cannot be reliably bound using most thermal activated adhesives. Further, such milling results in the production of debris that must be removed and disposed of during the subsequent binding process.
There is a need for an apparatus for conditioning a stack of sheets, prior to binding, that will permit the stack to be reliably bound using conventional thermal adhesive binder strips as previously described. As will be apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings, the present invention meets these and other requirements. Once a stack of coated sheets has been conditioned in accordance with the present invention, a reliable bind can be achieved using conventional relatively low cost desktop binding equipment and binder strips.